The quoted translation appears to be the James Legge translation. Here is Legge's translation of Chapter 1 of The Tao-te Ching:
The Tao that can be trodden is not the enduring and unchanging Tao. The name that can be named is not the enduring and unchanging name.
(Conceived of as) having no name, it is the Originator of heaven and earth; (conceived of as) having a name, it is the Mother of all things.
Always without desire we must be found,
If its deep mystery we would sound;
But if desire always within us be,
Its outer fringe is all that we shall see.
Under these two aspects, it is really the same; but as development takes place, it receives the different names. Together we call them the Mystery. Where the Mystery is the deepest is the gate of all that is subtle and wonderful.
It may be useful to compare that translation to other translations. Here is Derek Lin's translation with my emphasis on the part that appears to correspond with the quote in question:
The Tao that can be spoken is not the eternal Tao
The name that can be named is not the eternal name
The nameless is the origin of Heaven and Earth
The named is the mother of myriad things
Thus, constantly without desire, one observes its essence
Constantly with desire, one observes its manifestations
These two emerge together but differ in name
The unity is said to be the mystery
Mystery of mysteries, the door to all wonders
Using these two translations it appears that Lao-tzu is trying to say that we can observe the Tao in two different ways, with desire and without desire. However, what we observe in these two ways should not be viewed as opposites. Rather what we observe have similar aspects and emerge together. Together, what we observe is a unity and can be called "mystery".
If we relate with desire we will only see the surface manifestations of the Tao. If we relate without desire we may see the essence of the Tao. Either way of observing, it is still the Tao.
Lao-tzu. Derek Lin, translator. Tao Te Ching Online Translation. Retrieved on August 10, 2019, from https://taoism.net/tao/tao-te-ching-online-translation/
Lao-tzu. James Legge, translator. The Tao Te Ching. Retrieved on August 10, 2019, from http://classics.mit.edu/Lao/taote.1.1.html